1. Field of the Invention
The present invention relates to a method and an apparatus for local measurement of an icing factor for atmospheric air containing supercooled water.
2. The Prior Art
Measurements of icing factor are used In particular, but not exclusively, within the fields of aviation and navigation, wherein icing can be a substantial safety hazard and can be the cause of crashes.
The problem arises as a result of atmospheric precipitation, such as rain and mist, that has in its supercooled state a known propensity to freeze on to objects, such as hull and machine parts on vessels as well as aeroplanes.
Thus, in practice critical weather conditions have often necessitated manual observations with a view to forming an impression of the acute risk of icing in order to enable warning of aeroplanes and vessels, if necessary.
It is a problem in this context that such manual observation is based on a subjective discretion, and that consequently a standardised value for the risk of icing is not accomplished. Thus, a warning based on such subjective discretion will be associated with a good deal of insecurity, and the person who receives the warning cannot readily form a reliable picture of the risk of icing, except by relying on the recommendation of the issued warning.
Accordingly, U.S. Pat. No. 4,730,485 and published NO patent application Ser. No. 151,060 teach apparatuses configured for carrying out a more standardised measurement of the amount of atmospheric precipitation.
Thus, U.S. Pat. No. 4,730,485 teaches a stationary measurement device configured for measuring wind velocity as well as wind direction, but it is also suitable for measurement of the current icing.
Published NO patent application Ser. No. 151,060 discloses a stationary weighing apparatus for weighing an amount of atmospheric precipitation in the form of snow or ice that settles on a substantially horizontal weighing plate.
However, it is a problem in connection with these prior art devices that icing often occurs in situations when the air is relatively still, and that the icing factor measured by such devices are erroneous since only small amounts of ice are deposited on the devices.
It is a further problem in connection with the prior art device known from published NO patent application Ser. No. 151,060 that atmospheric precipitation other than supercooled water may deposit on the weighing plate and thus an erroneous weighing results in relation to the icing factor.
U.S. Pat. No. 3,940,622 discloses a detector for detecting the rate of ice-formation. In order to provide an in-stream of air, ambient air flow is created by inducing a primary gas into a duct, the primary gas being supplied, for example, by the engine compressor of a VTOL aircraft.
U.S. Pat. No. 5,317,915 is concerned with the problem of detecting icing-up of a rotor of an aircraft. This known measuring device comprises means for determining an axial load on the rotor axis under dynamic circumstances, i.e. during operation of the rotor.
It is therefore the object of the present invention to provide a method and an apparatus whereby it is possible in almost all weather conditions, in particular also light winds and still air, to provide a standardized measurement result or a standardized value for the icing factor.
The method according to the present invention thus comprises the following process steps, wherein at least one surface element is provided that is made of a material suitable for ice in atmospheric air to freeze on, said element having a predetermined surface area, wherein the surface element(s) is/are brought to a temperature that corresponds essentially to the temperature of the atmospheric air, wherein a relative movement at a predetermined velocity is subsequently created between the atmospheric air and the surface element(s) by allowing the surface element(s) to move through the atmospheric air, and or a predetermined period of time, and wherein the thickness or mass of the ice frozen fast to the surface element(s) is subsequently measured by means of a measurement device configured therefore after said predetermined period of time.
This means, on the one hand, that atmospheric precipitation other than supercooled raindrops or mist can only with difficulty settle on the surface elements and adversely influence the measurement results, and that it is hereby possible to perform relatively accurate measurements of the icing factor, also in relatively still air.
In case the factor measured is the thickness of the ice layer, it is advantageousxe2x80x94to obtain a representative value thereforexe2x80x94to measure in a number of points, preferably in one or more points on each surface element, the measurement results subsequently being summarised to a single value.
The value thus measured for the thickness of ice layer or weight increase will be a standardised factor that indicates a relative risk of icing of eg aeroplanes or vessels in the area in which measurement is performed. It goes without saying that the value can only be indicative of the risk since, obviously, it will vary more or less compared to the place where the measurement was performed.
The method can advantageously be exercised by use of an apparatus including at least one surface element made of a material suitable for ice in atmospheric air to freeze theron, the surface element(s) having a predetermined surface area, and wherein the apparatus further includes means for moving the surface element(s) through the atmospheric air at a predetermined velocity and for a predetermined period of time; and wherein means are provided for measuring the thickness or the mass of the ice frozen fast onto the surface element(s) after the predetermined period of time during which the surface element(s) have been moved through the atmospheric air.
According to a preferred embodiment of the method, it is ensured that frozen-on ice, if any, is removed prior to a first measurement, and likewise the frozen-on ice is removed from the individual surface element following measurement of its mass or thickness, whereupon a renewed measurement process can be performed. Advantageously, such removal of the ice can be accomplished by heating of the individual surface elements either from the outside or from the inside.
Further advantageously, uniform measurement results are accomplished if a cover is provided that extends at least across the surface element, and covers and shields the surface element, and said cover being removed from the surface element at least for the predetermined period of time during which the surface element is moved through the atmospheric air at a predetermined velocity.
The accuracy of measurement is further enhanced if the surface element is moved through the atmospheric air at a velocity that ensures that atmospheric precipitation that does not freeze on to the surface element is to a substantial extent thrown off the surface element.
In order to ensure that the inside of the cover is protected against atmospheric precipitation settling thereon, it can advantageously be so arranged in its second position that its inside is protected against atmospheric precipitation. It is hereby ensured that atmospheric precipitation cannot drip from the inside of the cover and onto the surface elements when the cover is conveyed across same.
In order to ensure that the surface elements are essentially free of other atmospheric precipitation prior to the amount thereof being determined, they can advantageously be rotated for a predetermined period of time following return of the cover to its first position.
According to a particularly simple method, at least two surface elements are used that are rotatably arranged on a rotor shaft whereby the movement of the two surface elements is effected by a rotation thereof about the rotor shaft.
According to a particularly simple embodiment, the apparatus comprises a weighing device configured for weighing and recording at least the weight of the surface element prior to and after movement of the surface element through the atmospheric air.
According to a particularly simple embodiment the apparatus comprises a rotor with a rotor shaft, and at least two surface elements that extend from the rotor shaft and protrude there from, and wherein means are configured for rotating the rotor about its axis. Hereby it is obtained that the movement mechanisms that bring about the movement of the surface elements through the air can be accomplished in a very simple manner that does not require maintenance.
Besides, with a view to also obtaining an increased accuracy of measurement the apparatus can also comprise a cover that is configured for assuming a first position in which it extends across the surface element, and thereby covers this upwardly, and a second position in which the cover has been removed from the surface element and does not cover same, and this cover is preferably configured such that in its first position, it forms a closed space around the surface element.
In order to be able to perform measurements in quick succession, means are conveniently provided that are, on the one hand, able to heat the surface elements in order to melt the ice deposited thereon, and on the other, to cool them to approximately ambient temperature. This can be obtained in that the surface elements are configured with passageways; and that the apparatus comprises means whereby air can be conveyed through the passageways either in the form of heated air or air with approximately ambient temperature. Heating and cooling of the surface elements can also be accomplished by the closed space underneath the cover being heated and cooled.
It is desirable that the ice formation on the surface elements is as comprehensive as possible to ensure accuracy and speed of measurement, and consequently the surface element will, in a first preferred embodiment, be in the form of a plate with a front and a back which have opposite orientations relative thereto; and wherein the plate is configured in such a manner that the front of the plate faces in the direction in which the surface element is moved through the atmospheric air; and whereinxe2x80x94through said platexe2x80x94a plurality of passageways are provided from the front of the plate to the rear of the plate, whereby atmospheric air is able to flow through the passageways from the front of the plate to the back of the plate.
According to an alternative, preferred embodiment the apparatus comprises a system of surface elements mounted on a rotatable shaft configured for being arranged in a substantially vertical position. The individual surface elements are configured and arranged such that the individual surface elements will, in correspondence with their projection on a face perpendicular to the rotatable axis, abut on or overlap other surface elements, which means that there is no space between the individual surface elements when the apparatus is viewed from above. Hereby it is obtained that all atmospheric precipitation within the expanse of the apparatus hits the surface elements and thus can be deposited in the form of ice. The larger the overlap between the individual surface elements, the larger the deviation from vertically falling precipitation can be tolerated while ensuring this.
In correspondence with the above teachings, the surface elements can advantageously be configured and arranged such that the individual surface elements corresponding to their projection on a face parallel with the rotatable axis abuts on or overlaps other surface elements, so as to accomplish that there is no space between the individual surface elements when the apparatus is viewed from the side. Hereby it is obtained that the atmospheric air conveyed across the surface elements by the relative movement between the atmospheric air and the surface elements hits a surface element and is thereby able to deposit the water contained therein in the form of ice.
In case of embodiments like the ones described above, it is ensured that the apparatus can be configured with the smallest possible physical dimensions.
The apparatus according to the present invention is particularly suitable for use in airports where the apparatus is preferably arranged at ground level and whereby the apparatus includes means for recording the measured results of the thickness or mass of the ice deposited on the surface elements, and means for visually or auditively emitting a signal regarding the measurement results to the monitoring personnel of the airport.